Porphyrin chemodosimeters: synthesis, electrochemical redox properties and selective ‘naked-eye’ detection of cyanide ions

Abstract

Dicyanovinyl appended β-pyrrole substituted porphyrins have been synthesized and utilized as chemodosimeters for ratiometric and colorimetric sensing of cyanide ions. In addition to meso-tetraphenylporphyrin (TPP) based system, we have designed and synthesized octaphenylporphyrin (OPP) based moderately nonplanar systems. These systems also depicted effective π-conjugation between the pyrrole ring and its olefinic substituent which is reflected in the UV-vis absorption spectra as the red-shifting of both Soret and Q-bands and possibly the splitting of the absorption Soret band. This article describes the single crystal X-ray structure, electronic spectral and electrochemical redox properties of these porphyrin chemodosimeters. The stable anionic-porphyrin species formed after addition of CN⁻ ions has a blue shifted unperturbed absorption spectrum due to the disruption of electronic resonance and charge-transfer interactions between the β-dicyanovinyl acceptor and porphyrin core. The probes exhibited excellent selectivity towards CN⁻ over other anions such as F⁻, Cl⁻, Br⁻, NO₃⁻, H₂PO₄⁻, AcO⁻, I⁻, HSO₄⁻, ClO₄⁻ and PF₆⁻. The action of the chemodosimeters were investigated by ¹H-NMR titrations, ESI-mass spectrometry, electrochemical studies and DFT calculations which supported the formation of cyanide addition adduct (porphyrin–CN)⁻. These chemodosimeters depicted lowest detection limit (LOD) of 0.023–0.082 ppm, which is very much lower than the permissible limit (0.2 ppm) and other porphyrinoids reported till date. These porphyrins are also able to detect cyanide ions in (9 : 1, v/v) CH₃CN : H₂O mixture with low detection limits thereby providing practical applicability of these chemodosimeters.